The invention relates to a ceramic cooktop comprising a cooking plate of glass ceramic or glass, an electric heat conductor layer and an insulating layer between the cooking plate and the heat conductor layer. The invention further relates to a method of producing such a ceramic cooktop.
Such a ceramic cooktop is for instance known from DE 31 05 065 C2 or from U.S. Pat. No. 6,037,572.
The known ceramic cooktop comprises a cooking plate of a glass ceramic, at the lower side of which a grounded metal layer is sprayed, onto which an insulating layer of aluminum oxide is sprayed. At the lower side of the ceramic insulating layer a heat conductor is applied by a printing technique.
Such a ceramic cooktop can provide a more energy saving heating than with previously known ceramic cooktops, wherein heating is substantially performed by means of irradiation energy. Herein initial cooking power is considerably enhanced.
The insulating layer between the heat conductor layer and the cooking plate is necessary, since a glass ceramic, such as Ceran®, comprises an NTC characteristic, i.e. with rising temperature also the electric conductivity raises considerably.
Therefore, the electric insulating layer must have a breakdown resistance of about 3,750 Volts at operating temperatures, to guarantee the necessary safety requirement according to VDE.
To this end it is necessary to produce the ceramic insulating layer with a considerable layer thickness, such as for instance 200-500 μm, when utilizing Al2O3 as insulating layer.
However, it has been found that the ceramic material tends to fracture formation at such a high layer thickness and, in addition, the thermal stresses that result from the differences between the coefficients of thermal expansion between glass ceramic (±0.15×10−6 K−1) and ceramic (≈8.0×10−6 K−1 for Al2O3) considerable thermal stresses result during operation, so that the ceramic insulating layer tends to chip off.